Recent studies implicate immediate early genes (IEGs) in drug addiction. Our laboratory has focused on an IEG termed Homer1a that is rapidly induced by cocaine. A family of Homer proteins has now been identified that bind receptors for group 1 metabotropic glutamate (mGluR) and inositol trisphosphate (IP3R). Homer proteins also interact with scaffolding proteins involved in NMDA receptor signaling. Cell biological studies support the hypothesis that Homer proteins function as an adapter system to regulate the coupling of intracellular calcium pools with membrane receptors, and may additionally play a role in receptor trafficking and synaptogenesis. The IEG form (Homer 1a) appears to function as natural dominant negative that disrupts crosslinking. This biochemical competition provides a mechanism by which natural synaptic activity can modify synaptic function. Studies conducted with the support of this grant have focused on generating mouse models to test the integrated cellular and systems-level functions of Homer. We generated transgenic mice that constitutively express Homer 1a, and used these mice to support the "dominate negative" hypothesis. We also generated knockouts (ko) of Homer 2 and 3, and will soon have a conditional ko of Homer 1. Initial analysis indicates that Homer 2 and 3 ko mice are viable. In Aim 1 we will use ko mice to examine the consequence of deleting Homer proteins. Because Homer proteins are biochemically redundant, our strategy includes generating mice with combined deletions of multiple Homer proteins. These mouse models will be the basis for understanding the contribution of crosslinking forms of Homer to both cellular (calcium signaling, trafficking and synaptogenesis) and systems-level (cocaine sensitization and learning) functions. Aim 2 will examine the function of the IEG form of Homer. We will use a new strategy that is designed to overcome limitations of current transgenic models, which exhibit mosaic expression of the transgene. Homer 1a will be expressed using a bacterial artificial chromosome (bac) of CaMKII. In parallel, we will generate a selective deletion mutant for Homer 1a. These mouse models will be used in physiological and behavioral studies to test and refine the dominant negative model of Homer IEG function. Thus, experiments in Aim 1 will tell us what crosslinking Homer proteins do, while Aim 2 will tell us what aspects of Homer function can be modulated in response to neuronal activity. Together, these studies will provide fundamental insight into how stimuli such as cocaine can effect long-term changes in synaptic and brain function.